Utility of Immunohistochemistry With Antibodies to SS18-SSX Chimeric Proteins and C-Terminus of SSX Protein for Synovial Sarcoma Differential Diagnosis.

ABSTRACT

Synovial sarcoma is a relatively common soft tissue tumor characterized by highly specific t(X;18)(p11;q11) translocation resulting in the fusion of SS18 with members of SSX gene family. Typically, detection of SS18 locus rearrangement by fluorescence in situ hybridization or SS18 SSX fusion transcripts confirms the diagnosis. More recently, immunohistochemistry (IHC) for SS18-SSX chimeric protein (E9X9V) and C-terminus of SSX (E5A2C) showed high specificity and sensitivity for synovial sarcoma. This study screened a cohort of >1000 soft tissue and melanocytic tumors using IHC and E9X9V and E5A2C antibodies. Three percent (6/212) of synovial sarcomas were either negative for SS18-SSX or had scattered positive tumor cells (n=1). In these cases, targeted RNA next-generation sequencing detected variants of SS18 SSX chimeric transcripts. DNA methylation profiles of 2 such tumors matched with synovial sarcoma. A few nonsynovial sarcoma tumors (n=6) revealed either focal SS18-SSX positivity (n=1) or scattered positive tumor cells. However, targeted RNA next-generation sequencing failed to detect SS18 SSX transcripts in these cases. The nature of this immunopositivity remains elusive and may require single cell sequencing studies. All synovial sarcomas showed positive SSX IHC. However, a mosaic staining pattern or focal loss of expression was noticed in a few cases. Strong and diffuse SSX immunoreactivity was also seen in epithelioid sclerosing osteosarcoma harboring EWSR1 SSX1 fusion, while several sarcomas and melanocytic tumors including cellular blue nevus (5/7, 71%) revealed focal to diffuse, mostly weak to intermediate SSX staining. The SS18-SSX and SSX IHC is a useful tool for synovial sarcoma differential diagnosis, but unusual immunophenotype should trigger molecular genetic testing.